There are many industrial process that use process water in carrying out reactions, as an effluent for removing unwanted by-products, as a diluent, and for many other functions. Examples of industrial processes, which use process water, include, for example, the refining of petroleum; the production of olefins, polymers, and organic acids; the production of metals, e.g. aluminum, iron, steel, and copper; and the benefaction of coal.
The process water often comes into contact with a variety of contaminants when the industrial process is carried out. These contaminants remain in the process water. Although there may be many contaminants in the process water and they vary depending upon the type of industrial process carried out, the more deleterious contaminants include suspended solids, oil and grease, metals, and silicate compounds.
The process water is often subject to elevated temperatures. It may be converted to steam, which often undergoes condensation. The condensate may also contain the contaminants that are present in the process water.
Although there are many methods known for removing contaminants from aqueous systems, these methods cannot be successfully used to remove contaminants from process water and condensates, particularly without reducing the heat capacity of the process water and/or condensate. The temperature of the condensate typically ranges from about 80° C. to 100° C., most typically from 95° C. to 100° C. What makes it difficult to purify the condensate is the presence of suspended solids, which can be 1000 times as high as that found in other contaminated aqueous systems. Because the temperature is elevated, it is difficult to purify condensate, particularly without reducing the heat capacity of the condensate. Additionally, the difficulty is compounded because the condensate may have high alkalinity, which increases the stability of the emulsion of oil found in the process water and/or condensate.
The elevated temperature and high alkalinity of the condensate also impairs the usefulness of chemicals typically used to break the emulsion, and/or coagulate suspended solids. Thus, many processes that could be used to purify condensate are not compatible with the high temperatures and alkalinity.
The temperature of condensate typically ranges between 80° C. and 100° C. If the purification can be carried out without any reduction in the heat capacity of the condensate, a great deal of energy can be conserved. The water does not have to be reheated for use in the process or as boiler feedwater.
One example of process water and/or condensate, which has the potential for reuse, is that generated by the production of alumina from bauxite ore. The majority of aluminum produced today is manufactured from bauxite ore. One of the primary means for converting bauxite ore to alumina is by the Bayer process as shown in FIG. 1. The alumina is then converted to aluminum, which is produced commercially by the electrolytic smelting of alumina.
The Bayer process for purification of bauxite ore into alumina involves the high temperature digestion of the bauxite ore in a solution of sodium hydroxide (caustic). The digestion typically takes place at 100 to 300 psi. The effluent from the digestion is flashed, i.e. reduced in pressure, in eleven stages to atmospheric pressure. Each step produces steam as the pressure drops. This steam is fed into a heater coil in the next immediate downstream vessel to condense the steam into process water and/or condensate. This condensate is often waste because contains small amounts of aluminum, iron, silica, caustic, and organics. The contamination is caused by carryover of effluent liquor into the flashed steam. The contamination contains both soluble and insoluble material. The insoluble material is referred to as “red mud”.
Both the red mud and the dissolved material are present in the process water and/or condensate at varying amounts depending upon various operating conditions. Often an antifoam is used to keep high froth levels from increasing carryover. The antifoam may contribute to the organic contamination in the condensate. The typical alumina plant will produce thousands of gallons per minute of this condensate. It is often wasted, but could be used for boiler make up water if the purity were improved. This could result in millions of dollars saved each year at each plant site.
For purposes of describing this invention, condensate is condensate that results from the condensation of steam generated from any stage of the process whereby bauxite is converted to alumina, particularly the Bayer process. There are three major sources of condensate in an alumina facility. There is the digestion condensate that is the most contaminated, the evaporator condensate which is somewhat contaminated, and the clean condensate from surface condensers and the like (closed systems with no process contact). The condensate carries impurities such as mineral oil, silica, iron oxide, aluminum and other suspended solids from the ore. Because condensate usually contains some of the caustic from the digestion process, the oil can be strongly emulsified and the aluminum dissolved. The pH of the condensate can vary over wide ranges, but it highly alkaline. The pH is typically 10.0 to 11.0.
Because the temperature of the condensate is typically from about 95°-100° C., it has the potential to be used as a boiler feedwater if the impurities could be removed. However, if utilized without treatment, the boilers would exhibit frequent failures, which would result because of the precipitation of impurities. Because there is no effective and economical way of removing the impurities from the condensate, the condensate is frequently wasted.
All citations referred to in this application are expressly incorporated by reference.